1. Field of the Invention
The present invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device with an ingenious patterning of an electrical insulating film or a metal film formed on the top face of a semiconductor chip.
2. Description of the Related Art
An AlGaInAs/InP-based laser, which exhibits good thermal and high-frequency characteristics, has been spotlighted as a semiconductor laser for optical fiber communications. Another AlGaInP/GaAs-based laser, which exhibits good thermal characteristics and excellent high-power operation, has been also spotlighted as a semiconductor laser for information processing. Most of these semiconductor lasers employ a ridge waveguide type structure, which can reduce processing steps and costs for manufacturing them.
In such ridge waveguide type semiconductor lasers, stress due to patterning of an electrical insulating film or an electrode formed on the surface of the chip are considered for one of degradation factors during energizing.
The related prior art is listed as follows: Japanese Patent Unexamined Publications (kokai) JP-A-2002-280663.
FIG. 10 is a plan view showing a conventional AlGaInAs/InP-based semiconductor laser device of ridge waveguide type. FIG. 11 is a perspective view showing the laser device in FIG. 10.
On the top face of a laser chip 1 formed are two grooves 12 by etching, and between these grooves 12 formed is a narrow ridge portion 11. On both sides of the grooves 12 formed are two stage portions 13 and 14, respectively.
The top face of the laser chip is covered with an electrical insulating film 3, such as SiO2, except the vicinity of the summit of the ridge portion 11, on which a stripe-like opening is provided. On the upper faces of the stage portions 13 and 14 formed are underlaid films 4 and 7, such as SiO2. On the underlaid film 4 of the stage portion 13 formed is a deposited electrodes, and on the deposited electrode 5 a plated electrode 6 is formed. Meanwhile, another deposited electrode 8 is formed so as to extend from the underlaid film 7 of the stage portion 14 via the groove 12 to the summit of the ridge portion 11, and on the deposited electrode 8 a plated electrode 9 is formed. Both the deposited electrode 8 and the plated electrode 9 have a chip contact portion 9a on the upper face of the ridge portion 11, a lead-out portion 9b in the groove, and a bonding pad 9c on the upper face of the stage portion 14.
Next, problems on the conventional semiconductor laser device of ridge waveguide type will be described below. After continuously energizing the semiconductor laser device for a long time, the device is gradually degraded, finally laser oscillation will be stopped. For one approach for analyzing degradation factors of such a device, bottom face EL (electro-luminescence) evaluation is known. The bottom face EL evaluation is a method of observing light-emitting conditions of a laser device from the bottom face of the chip. In case crystalline defects occur in the chip during energizing, dark spots or lines can be observed, thereby effectively detecting degraded locations or degradation factors in the waveguide.
FIGS. 12A and 12B are explanatory views illustrating a relation between a pattern on the surface of the chip and an EL pattern on the bottom face. The EL pattern on the bottom face, as shown in FIG. 12B, is a macrophotograph which is taken by picturizing a light-emitting pattern of a degraded device from the bottom face of the chip, after continuously energizing it, e.g., for several hundred hours with a current of one hundred and several dozen mA at a temperature of 85 degree-C.
As seen in detail from the EL pattern on the bottom face, typical dark spots occur (1) at a location corresponding to a front edge of the chip-contact portion 9a of the plated electrode 9, (2) at a location corresponding to front edges of the underlaid films 4 and 7, and (3) at locations corresponding to the lead-out portion 9b of the deposited electrode 8 and the plated electrode 9, and a front edge of the bonding pad 9c, respectively. Each of these front edges and each of the locations in which the dark spots occur commonly reside on a straight line perpendicular to the longitudinal direction of the ridge portion 11.
Firstly, a main degradation factor in item (1) will be discussed below. Thickness of the plated electrode 9 is typically about 4 μm, whereas thickness of the deposited electrode 8 is typically about 0.8 μm, thereby causing a higher current density in the vicinity of the edge of the plated electrode inside the deposited electrode. The current concentration on the vicinity of the edge of the plated electrode causes a localized rise of temperature, so that crystalline defects occur inside an active layer right under the edge of the plated electrode. This is one of degradation factors.
Secondly, main degradation factors in items (2) and (3) will be discussed below. As referring to FIG. 10, the underlaid films 4 and 7, the deposited electrodes 5 and 8, and the plated electrodes 6 and 9, which are formed on the surface of the chip, have linear portions W1, W2, W3, W4 and W5, perpendicular to the ridge portion 11. When the ridge portion 11 is supplied with a current, the temperature of the whole chip rises up, not only in the ridge portion 11 where the current passes through, but also in an area where no current flows. The rise of temperature of the chip causes stress, since a crystal constituting the laser chip, a material of the underlaid films and a material of the electrodes have different coefficients of thermal expansion. In case the ridge portion 11 undergoes such stress from the linear portions perpendicular to the ridge portion 11, crystalline defects occur inside an active layer right under the ridge portion 11, thereby causing dark spots in the bottom face EL pattern, as shown in FIG. 12B. In particular, the lead-out portions of the deposited electrode 8 and the plated electrode 9 is located closely to the ridge portion 11, causing larger stress.
Further, as to another degradation factor of item (1), in addition to the above-mentioned current concentration in the vicinity of the edge of the plated electrode, the linear portion w6 of the edge of the plated electrode intersects perpendicularly to the ridge portion 11, thereby causing stress due to the difference in coefficient of thermal expansion between the metal constituting the electrode and the crystal. This may be one of degradation factors.
As described above, in the conventional semiconductor laser device of ridge waveguide type, because of stress due to the differences in coefficient of thermal expansion among the electrical insulating film and the electrodes, which are formed on the surface of the chip, and the crystal, crystalline defects occur during energizing, thereby presumably causing the device to be degraded.